Hypnotherapy is currently unregulated in the UK meaning that anyone can set up as a Clinical Hypnotherapist regardless of the training, if any, that they have received. I have met one who was issued with a Diploma in Clinical Hypnotherapy after a single weekend course!
One safeguard you do have is to select a hypnotherapist who is registered with the Complementary and Natural Healthcare Council (CNHC) website
Even with specialist knowledge of the subject it can be difficult to ascertain the competence of a Clinical Hypnotherapist or the quality of their training. In making your selection do not be afraid to ask for information about them or to contact their professional body to confirm their status. If any Hypnotherapist is evasive, vague, fails to answer your questions, refuses to do so or cannot give a proper account of their qualifications and the training that led to the award of their qualifications then exercise caution; my details are fully checkable via the British Society of Clinical Hypnosis (BSCH)
This link will take you to the BSCH's information about choosing a hypnotherapist "Finding a Good Hypnotherapist"
If you live outside my area then this link will allow
you to find selected, highly qualified Hypnotherapists in other areas
of the UK - other Hypnotherapists